As wireless communications has become prevalent in society, it is not uncommon for two wireless communications devices to interfere with the operation of each other when operating within transmission range of each other. Indeed, this type of interference has become more frequent with increasing wireless connectivity permeating society, such as, cellular phones and mobile email devices. For example, when IEEE 802.11b (WiFi) wireless devices, for example, laptop computers, became readily available to the home consumer, “cordless” home telephones were capable of rendering the WiFi devices inoperable due to interference.
Interference is typically categorized into two types: narrowband interference and wideband interference. Narrowband interference includes forms of interference that are concentrated around a relatively small frequency bandwidth. Several common approaches to compensating for narrowband interference may include, for example, forward error correction (FEC), baseband coding (FHSS/DSSS spread spectrum), diversity in frequency and/or time, and signal cancellation.
Differently, wideband interference includes forms of interference that are spread out across a relatively large frequency bandwidth. Several common approaches to compensating for wideband interference may include, for example, FEC, adaptive data rate (modulation and channel bandwidth), adaptive power control, diversity in frequency and/or time, and signal cancellation.
Another common type of interference is self interference. Common causes of self interference may include, for example, collocation or frequency reuse. In particular, collocation may be caused by two wireless communication systems being deployed too closely together while frequency reuse may be caused by two wireless communication systems using the same frequency band/channel. Several approaches to compensating for self interference, which are available during planning and development stages of wireless communication systems, include, for example, installation (isolation between antennas), frequency planning, antenna pattern manipulation, synchronization of transmitters, and filtering (duplexing).
Although interference is commonly generated inadvertently, another frequent source may be intentionally created by an unauthorized user, e.g. an interferer. Indeed, similar to the cordless home telephone, the unauthorized user may render a wireless communication system inoperable or may reduce the effectiveness, for example, capacity, of the wireless communication system.
Regardless of the source, there is a desire to reduce the effect of interference in communications. An approach to interference in wireless communication systems may be interference compensation. For example, frequency diversity may be used to compensate for the interference by adjusting the operating frequency of the wireless communications device to a frequency free of the interference. Another approach to interference compensation is spatial diversity using a plurality of antennas. In this approach, the undesired interference is canceled out using the time shifted variants received at each antenna.
Another approach to interference compensation is disclosed in U.S. Pat. No. 6,115,409 to Upadhyay et al. This system includes an array of antennas for spatial diversity. The system uses the spatial diversity to attempt to cancel out the interference. Moreover, the system uses spatial and temporal filters to cancel out wideband and narrowband interference in the antenna array.
Another approach to interference compensation is disclosed in U.S. Pat. No. 7,039,417 to Lyle et al. The system of Lyle et al. cycles through available channels and evaluates the presence of interference on each channel. The system may also determine the type of electronic device causing the interference, i.e. a cell phone or a microwave. The system assigns a quality-of-service parameter for the channel and moves on to the next channel. Using the table of service parameters for each available channel, the system may select a channel that is interference free or subject to low levels of interference. In other words, this system uses frequency diversity to compensate for interference.
Another approach to interference compensation is disclosed in United States Patent Application Publication No. 2008/0043861 to Moffatt, which is assigned to the present application's assignee, the contents of which are hereby incorporated by reference in their entirety. This approach comprises a device for communicating data, comprising a modulation and mapping circuit that modulates and maps data symbols into a plurality of multiple subcarrier frequencies that are orthogonal to each other to form an Orthogonal Frequency Division Multiplexed (OFDM) communications signal based on a fixed or variable OFDM symbol rate. A pseudo-random signal generator is operative with the modulation and mapping circuit for generating pseudo-random signals to the modulation and mapping circuit based on an encryption algorithm for frequency hopping each subcarrier at an OFDM symbol rate to lower probability of interception and detection, reduce power per frequency (dB/Hz/sec), and lower any required transmission power while maintaining an instantaneous signal-to-noise ratio.